The present invention relates to a transmitter, a wireless transmission system as well as to a method for the transmission of data symbols over a communication channel which particularly finds its application in the field of so-called CDMA systems.
CDMA (Code Division Multiple Access) transmitting systems are known from the state of the art. According to one CDMA technique, after the modulation (symbol mapping), the symbols are spread by a so-called spreading sequence or spreading code. After spreading the resulting data stream is scrambled by a scrambling sequence of a scrambling code. The thus resulting data stream, which has been spread and scrambled, is then power-amplified and sent over a communication channel. The reverse procedure happens at the receiving side.
In FIG. 3 an example for a transmission system comprising scrambling and spreading is shown. In the example of FIG. 3 only the downlink communication channel 26 from a base station 24 to a mobile station 25 is shown. The downlink 26 can comprise different channels D1, . . . , DN. Each channel comprises channelisation (spreading) 28, 30 and scrambling 29, 31. According to the state of the art in one link, as for example the downlink 26 as shown in FIG. 3, only one scrambling sequence (scrambling code) and several different spreading codes (often referred to as channelization codes) are used depending upon the data rate and services required. The drawback of this approach is that only certain types of channelisation codes can be used together and the highest data rate is constrained by the length of the shortest channelisation code. This is especially true when codes of different rates are used.
From WO96/05668 A1 and EP-A-565 506 techniques for multiple access coding for radio communication is known. According to these documents information symbols are spread using orthogonal or bi-orthogonal codewords. This spread information symbols are assigned a unique scramble mask that is taken from a set of scramble masks having selected correlation properties. The set of scramble masks is selected such that the correlation between the modulo-2 sum of two scramble masks with any codeword is a constant magnitude independent of the codeword and the individual mask being compared. According to one embodiment of WO 96/05668 A1, when any two masks are summed using modulo-2 sum arithmetic, the Walsh transformation of that sum results in a maximally flat Walsh spectrum. For cellular radio telephone systems using subtractive CDMA demodulation techniques, a two-tier ciphering system ensures security at the cellular system level by using a pseudorandomly generated code key to select one of the scramble masks common to all of the mobile stations in a particular cell. As according to these techniques one common scramble mask is used for all mobile stations in a particular cell, the above-cited drawback arises that only certain types of channelisation codes can be used together limiting the number of available channelisation (spreading) codes.
In view of the above-cited drawbacks it is of the object of the present invention to enhance the flexibility of code assignment for a CDMA system, particularly within a cell sector and/or to increase the maximum data rate.
The central idea of the present invention thereby is a new code allocation scheme for a CDMA system utilising two or more scrambling codes within one link (uplink or downlink).
According to the present invention therefore a transmitter for communicating data symbols over a communication channel is provided, the transmitter comprising means for spreading each data symbol with a respective spreading code, wherein the spreading codes are mutually orthogonal. Furthermore means for scrambling each spread symbol with a respective scrambling code are provided, the scrambling codes respectively having the same length as the spreading codes. Means are provided for the transmission of the spread and scrambled symbols. According to the present invention the means for scrambling are provided with a plurality of different scrambling codes which can be used simultaneously within the same link.
The means for scrambling the spread symbols can use different scrambling codes for an uplink communication channel and a downlink communication channel.
The spreading codes can be obtained by means of a code tree.
According to an aspect of the present invention, only in a downlink communication channel different scrambling codes are used, wherein in an uplink channel only one scrambling code is used.
The means for scrambling can use different scrambling codes within the same link only for channels demanding a high bit rate.
According to the present invention, furthermore a wireless transmission system comprising a transmitter of the above-cited type is provided, wherein a subset of scrambling codes is respectively allocated to a cell of the transmission system.
Different spreading codes can be allocated to adjacent cells of the transmission system.
According to the present invention, furthermore a method for the transmission of data symbols over a communication channel is provided. Data symbols are spread with a respective spreading code, wherein the respective spreading codes are mutually orthogonal. Each spread symbol is scrambled with a respective scrambling code, the scrambling codes not changing the band rate of the transmission (in contrast to the spreading codes). The spread and scrambled symbols are transmitted.
According to the present invention, when scrambling a plurality of different scrambling codes can be used, which are to be used simultaneously within the same link.
In the step of scrambling different scrambling codes can be used for an uplink communication channel and a downlink communication channel, respectively.
The spreading codes can be obtained by means of a code tree.
As an aspect of the present invention, only in a downlink communication channel different scrambling codes are used. In the uplink communication channel only one scrambling code is used, as the bit rate usually is less than in the downlink communication channel.
Different scrambling codes can be used within the same link only for channels demanding a high bit rate.
Particularly different scrambling codes within the same link can be used for example for video channels and/or data channels, but for example not for voice channels.